Nucleoside analogues have been developed as antiviral and anticancer agents. Nucleotide kinases phosphorylate nucleosides to their corresponding 5′-monophosphates which are further converted into their di- and tri-phosphates by cellular nucleotide kinases.
Some nucleosides are weakly active because they cannot be efficiently phosphorylated by kinases or are not substrates of kinases at all, as evidenced by the observation that some inactive nucleosides, when converted chemically to triphosphates, become potently active against certain viruses in vitro. Nucleoside phosphates (nucleotides) per se cannot be used as drugs often because they are de-phosphorylated by membrane nucleotides and/or other hydrolases before entering the cells or are too polar to enter the cells. To improve biological activity of nucleosides, their phosphate prodrugs have been intensively studied because they can potentially bypass the rate-limiting first step of phosphorylation. Recently, phosphoramidate prodrug approach has been reported to be an effective method to convert biologically inactive nucleosides to their active nucleoside monophosphates bypassing the rate-limiting first step of phosphorylation (see, e.g., J. Med. Chem. 2007, 50, 5463; WO 2008/121634; WO 2008/082601; and WO 2008/082602). In recent years, there are a number of patent applications reporting utilization of the phosphoramidates as prodrugs to deliver nucleoside monophosphates to tissues, such as liver (see U.S. Pat. No. 6,455,513; WO 2009/052050; WO 2008/121634; WO 2008/0833101; WO 2008/062206; WO 2007/002931; WO 2008/085508; WO 2007/095269; WO 2006/012078; and WO 2006/100439). The nucleoside monophosphates can be further phosphorylated to diphosphates and then the corresponding biologically active triphosphates.
However, the above-mentioned phosphoramidate approaches based on McGuigan's technology (U.S. Pat. No. 6,455,513) have various limitations due to potential neurotoxicity and liver and kidney damages caused by phenol released from prodrugs (Carcinogenesis 1993, 14, 2477; Mutat. Res. 1991, 249, 201).
McGuigan's phosphoramidate of nucleoside usually can demonstrate maximum biological activity in cell line assays because it can release nucleoside or nucleotide quickly in the cells. It was reported that phosphoramidate prodrug of d4T could not be detected in plasma after oral administration immediately (Drug Metab. Dispos. 2001, 29, 1035). Phosphoramidate is stable in gastric fluid and may be absorbed in the stomach. On the other hand, phosphoramidate may decompose readily in intestinal fluid to ala-d4T-MP which may not be absorbed efficiently in intestine due to its polar nature. Therefore, bioavailability of this type of phosphoramidate prodrugs usually is low, probably due to its hydrolysis catalyzed by esterase followed by releasing phenol. For example, the bioavailability of GS-7340, an isopropylalanyl monoamidate phenyl monoester of tenofovir, was only 17% in male beagle dogs (Antimicrob. Agents Chemother. 2005, 49, 1898). Phosphoramidate prodrug of 2′-C-methylguanosine only delivered about 10%-20% of active triphosphate delivered into the liver from free nucleoside (J. Med. Chem. 2010, 53, 4949).
Efforts to search for phosphate prodrugs that would be cleaved by an esterase independent mechanism have led to the discovery of HepDirect prodrugs (J. Med. Chem. 1994, 37, 498; J. Am. Chem. Soc. 2004, 126, 5154; J. Pharmacol. Exp. Ther. 2005, 312, 554). Erion et al. disclosed that cyclic phosphate or phosphonate prodrugs which are stable in the presence of esterase can enhance liver specific drug delivery (Erion, M., et al., U.S. Pat. No. 7,303,739 and reference thereof). Erion's prodrugs are activated by P450 enriched in the liver. However, clinical application of this approach may be limited by potentially adverse side effects caused by α,β-unsaturated ketone metabolites from prodrugs.
In the McGuigan's prodrugs, as shown in the general phosphoramidate structure of formula F1, Ra is aminoacid ester residue while Rb is an aryl group including phenyl or nathphyl. Rb would be simultaneously released after ester of aminoacid residue was hydrolyzed by esterase. Sofia (U.S. Pat. No. 7,964,580) reported only one phosphoramidate with Ra as L-alanyl isopropyl ester and Rb as phenyl group. Although Rb was defined as an alkyl group, such as Me, Et, iPr, and t-Bu, there is no phosphoramidate prodrug with Rb as alkyl group that has actually been prepared. WO 2012/142075 also reported phosphoramidates with Rb as benzyl groups with no substitution on phenyl group. However, no biological activity for any of such compounds was reported. Therefore, phosphoramidates with Rb as alkyl or benzyl group without substitution on phenyl ring have never demonstrated biological usefulness, probably because these groups (alkyl or benzyl group) in phosphoramidate prodrugs of nucleoside are too stable to be cleaved efficiently to produce active nucleoside phosphate.

Therefore, new prodrug forms of nucleoside or nucleotide compound are still being actively pursued.